Prior to the present invention, the concept of controlling a relay valve by fluid pressure communication and exhaust lines has been taught. For example, this relay valve control concept has been taught in WABCO Brochure 973001, issued in Feb., 1977. The particular relay valve taught in this brochure includes, at a position above its relay piston, a control chamber, and at a position below such relay piston, a release chamber. Such control chamber and release chamber are identified in the brochure as 7 and 6, respectively. By means of a control pressure fluid communication line, which also includes a control connection, identified as 4, the control pressure can be communicated into the control chamber 7, and such control pressure can be exhausted from this control chamber 7. On the one hand, the exhaust chamber is connected with a fluid pressure user circuit by means of an operating connection, identified as 2. On the other hand, in this prior art relay valve, the exhaust chamber can be connected to a pressure release chamber by way of an inlet valve, identified as 5, and a supply connection, identified as with a fluid pressure medium supply source, or by way of an outlet valve, identified as 6, and a pressure-release line, identified as 3. This prior art relay valve can be activated in a number of ways. For example, such relay valve will be activated by either supplying or exhausting a control pressure into, or out of, the control chamber 7 so that such relay piston may activate the relay valve accordingly. With this particular prior art relay valve, the same control pressure is being achieved in the relay valve as a consequence of the design of the relay piston. The design of the relay piston results in an equal control pressure in the exhaust chamber and in the consumer circuit after the operation of the valve.
As used in the present specification, the terms "User System Volume" and "Control Chamber Volume" are to be considered as including the volumes of their respective operating lines and of the outlet chamber and of the control pressure fluid communication line.
Now, for the purposes of understanding the problems associated with relay valve control that exist with the prior art control schemes, let us assume that the flow resistance of any existing operating line is such that at fully open valve units the user pressures that exist in the outlet chamber of the relay valve and in the user circuit will always be roughly equal. Normally, the user circuit volume is larger than the volume of the control chamber 7. Consequently, because of the flow resistance within the operating circuit or the above-described user circuit volume, the customary prior art relay valves will react, after a certain delay period, upon a control pressure command, which means: The user pressure reacts after control pressure command (in time).
This relationship is illustrated schematically in the pressure versus time graph of FIG. 2 of the drawings. Such schematic illustration of FIG. 2 indicates a pressure-increase phase on the left and a pressure-drop phase on the right. Parallel to the time axis, on the left-hand side of the graph, a NOMINAL.sub.1 line is for the desired first NOM user pressure and, on the right-hand side of the graph, an additional NOMINAL.sub.2 is illustrated in relation to the time axis, which depicts a termination of the pressure-drop phase of the desired second NOM user pressure. It is evident to those persons who are skilled in the fluid pressure control art that in the pressure-increase phase, the control phase reaches the NOM.sub.1 value at time t.sub.1, whereas the user pressure will attain this value only at a later time t.sub.2. The difference between the time t.sub.2 -t.sub.1 constitutes the delay time. Those skilled in the art will, likewise, recognize that in the pressure-drop phase, the delay period will be t.sub.4 -t.sub.3.
There are cases, which can be envisioned, where such delay will cause problems in operating systems. A case exists, for example, when the user circuit will consist of one or several brake cylinders for an anti-lock brake system on a vehicle. If we assume that the user pressure, which has been reached at t.sub.1, and the vehicle wheel or wheels, which are part of the user circuit, have attained the locking limit, then the anti-locking circuit of such anti-lock brake system emits at that time, a signal, which triggers a discontinuation of the control pressure. In spite of the disrupted control pressure, the user pressure will continue to rise until reaching the NOM.sub.1 value at a time t.sub.2. The delayed increase in user pressure in the delay time period t.sub.2 -t.sub.1 normally will lead to an undesirable high slip of the vehicle wheel, or it will at least require special steps for its avoidance. The same is true, for example, when the wheel at the first NOM user pressure has reached its locking limit and the anti-locking circuit initiates a pressure-release phase. This is shown in the right portion of the schematic illustration of FIG. 2. The result, in this particular case, is an inferior braking action instead of a high slip. In this event, consumption of fluid pressure will increase.